How to disable the face overlay in the UV editor?
Go to your User preferences and under Themes for the UV/Image Editor category, lower the alpha of Face Selected to suit your needs. At 0, only the outlines should still show.



It is also worth noting that there is another option for Face color in the same category, the default color for this is white and the default alpha is ~0.3.
Therefore, if you have alot of faces overlaying each other, this can lead to a white look as it will add up so you can also adjust the alpha value there as well depending on what you need. Setting both Face and Face Selected alpha values to 0 should suffice.